AP Photo/Michel Euler Britain's international partners are aghast, mostly in silence, at the possibility one of the leading Western powers could break up and turn in on itself if Scotland votes for independence from London.

Many fear such a split would foreshadow a British exit from the European Union, on which Prime Minister David Cameron has promised a referendum in 2017, weakening the world's sixth largest economy and its continental partners.

Some also fear it would set a precedent, boosting separatism in Catalonia and elsewhere in Europe. Independence campaigners from around the continent have underlined this by flocking to Edinburgh to support the "Yes" campaign in the Sept. 18 vote and to draw lessons for their own struggle.

Some posed for a group photograph with Scottish nationalist leader Alex Salmond, now racing to turn narrowing opinion polls into a win in the Sept. 18 vote.

Scotland's First Minister Alex Salmond (front C) poses with supporters of the 'Yes Campaign', in Edinburgh, September 9, 2014. REUTERS/Russell Cheyne The idea is not sitting well in leading capitals.

U.S. President Barack Obama has been forthright in urging Britons to stay together and in the EU.

"We obviously have a deep interest in making sure that one of the closest allies that we will ever have remains a strong, robust, united and effective partner," Obama said on a visit to Europe in June.

U.S. Navy Secretary Ray Mabus, while stressing it was up to the Scottish people to decide, underlined this week the importance of Scotland in building and basingBritain's nuclear submarine force.

"It would be unwise for an American to take sides in this, but there would be some issues that would require some serious work," he told a Reuters Summit on Wednesday.

Privately, U.S. and European officials say they are concerned that a Britain amputated of a third of its territory and 8 percent of its population would be more introverted and less willing to engage in military coalitions abroad.

While France has said it is willing to join Obama in air strikes against Islamic State militants in Iraq, Cameron has played for time and avoided a clear commitment so far.

Last year Cameron failed to win parliamentary support for joining proposed U.S. air strikes in Syria, reinforcing opposition in the U.S. Congress that eventually prompted Obama to abandon the plan.

Scotland Labour Party supporters protest in support of the "No" vote in the referendum. Jeff J Mitchell/Getty Images Puzzled

European partners have tried to be quietly supportive of Cameron on Scotland, underlining how keen they are to keep Britain in the EU, while privately puzzling over his decision to risk everything on two uncertain referendums.

Merkel has made clear Berlin wants to keep Britain in the EU and is willing to accommodate some of London's demands for changes to help Cameron win the in/out plebiscite he has promised if he is re-elected next year.

Publicly, she has avoided taking sides, saying: "There's a fair campaign taking place. So I have nothing to criticize. I also think the British are not waiting for advice from the German chancellor on what to do, let alone the Scots."

But German Foreign Minister Frank-Walter Steinmeier was less diplomatic on Thursday. "I would openly admit that I would rather see Great Britain remain together," he told a news conference in Berlin with his British counterpart Philip Hammond.

In highly centralized France, it is the precedent of separatism that is causing most political anguish.

"The idea that this can happen in Europe in regions like Scotland -- even though it isn't at all the same context -- demonstrates that frontiers are exploding and this is worrying," said Socialist lawmaker Patricia Adam, who chairs the parliament's defense committee.

A demonstrator carries a sign during a pro-independence march in Edinburgh, Scotland. AP Photo/Jill Lawless Some European diplomats are alarmed by Cameron's high-wire strategy for trying to hold the country together and in Europe. "How could he take such ill-calculated risks?" a French diplomat asked privately.

He pointed to France's own history of lost referendums that brought down President Charles de Gaulle in 1969, stopped a proposed EU constitution in 2005 and very nearly killed off Europe's single currency before it was born in 1992, when voters approved the Maastricht Treaty by a whisker.

If Scotland does vote for independence, its future status with the EU could be a matter of lengthy dispute.

Diplomats say the Commission was bound to support Britain, a member state, in its legal interpretation before the vote. But EU governments would be likely to take a more pragmatic approach to Scotland if the vote was won.

Scotland's independence could have implications far beyond the United Kingdom's borders. Jeff J Mitchell/Getty New States?

"If Scotland becomes an independent state and manages to get into the EU, it will not be the only one. There will be what we call a domino effect," he said, citing Spain's autonomous provinces of Catalonia and the Basque Country.

The Catalans are the most immediate issue but support for independence is much more limited in other parts of Europe such as Flanders in Belgium or the swathe of Northern Italy which separatists call Padania.

The main Flemish nationalist party, the N-VA, has played down its separatist agenda and is about to join a Belgian federal government for the first time. Support for independence in Flanders is only about 10-15 percent, said Dave Sinardet, a political scientist at the Free University of Brussels.

"The Scottish vote for us is an injection of enthusiasm and hope," said Alfred Bosch, spokesman for Catalonia's Republican Left party. "The difference is that the UK government has been democratic and allowed it, but the Spanish government hasn't."

Spanish Prime Minister Mariano Rajoy has been silent on Scotland but the head of the Catalan regional government, Artur Mas, has said a Scottish "Yes" would be positive for the independence movement.

Conservative Spanish commentators have slammed Cameron for allowing the Scottish vote. Respected columnist Jose Antonio Zarzalejos called it "one of the greatest risks for the EU and for Great Britain in 300 years".

"If secession in Scotland prospers because of Cameron's huge and historic clumsiness, the EU will jump into action so that similar situations are not repeated, and it will pressure governments to negotiate with the secessionist movements we already have," he wrote on elconfidencial.com.

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